1. Field of the Invention
This invention relates to the field of electronic circuits, and in particular to a level shifter with independent ground potentials, and reduced electromagnetic emissions (EME).
2. Description of Related Art
Level shifters are commonly used to provide communication of signals from a system having a first range of voltage potential to another system having a different range of voltage potential. For example, level shifters are commonly used in automotive applications, because typical automobile systems operate at 0-18 volts, whereas common integrated circuits typically operate at 0-3 volts, or 0-5 volts. An asserted xe2x80x9clogic-highxe2x80x9d three or five volt output from an integrated circuit may be insufficient to be interpreted as a logic-high in a twelve volt system. A level shifter provides the appropriate mapping of voltage levels from one system to another, to ensure the proper communication of logic values.
FIG. 1 illustrates an example schematic of a conventional level shifter 100 that is configured to provide a logic-high voltage level of Vd2 at its output (Out) corresponding to a logic-high voltage level of Vd1 at its input (In). The logic-low levels of both signal (In, Out) is zero volts. Each of these voltage potentials are referenced with regard to a common ground potential.
When a logic-high (Vd1) input signal is received at the In node, the transistor N1 is turned on. At the same time, the inverter 110 turns transistor N2 off. With transistor N1 turned on, and N2 turned off, transistor P2 is turned on, thereby pulling the Out node to Vd2. In this manner, an input voltage level of Vd1 is converted to an output voltage level of Vd2.
When a logic-low (ground potential) input signal is received, transistor N1 is turned off, and the inverter 110 turns transistor N2 on. With transistor N1 turned off, transistor P2 is turned off, and transistor N2 pulls the Out node to ground potential. In this manner, an input signal at ground potential provides an output signal that is also at ground potential.
Note that the level shifter of FIG. 1 relies on a common ground potential for proper operation. If the ground of the input system (inverter 110) differs from the ground potential of the output system (N1, P1, N2, P2), ground currents will flow. If the difference between ground potentials is significant, the level shifter 100 may fail to operate, or may operate intermittently.
FIG. 2 illustrates an example schematic of a level shifter 200 that is configured to convert signals from floating sources 210, 220 into signals for driving a bus between two potentials, V+, Vxe2x88x92, as presented in U.S. Pat. No. 6,154,061, xe2x80x9cCAN BUS DRIVER WITH SYMMETRICAL DIFFERENTIAL OUTPUT SIGNALSxe2x80x9d, issued Nov. 28, 2000 to Hendrik Boezen et al, and incorporated by reference herein. Transistor P2 and diode D1 are configured to drive a bus load Lb to a potential of V+, and transistors N2 and diode D2 are configured to drive a bus load La to a potential of Vxe2x88x92 and therefore substantially independent of potentials V+ and Vxe2x88x92 for both logic-high and logic-low signal levels. The level shifter 200 provides equal and opposite currents to the loads La, Lb, thereby avoiding ground currents, and ground bounce as the bus switches state. The output transistors P2 and N2, however, are hard-driven to each corresponding rail voltage, V+ and Vxe2x88x92, at each bus state transition, causing substantial electromagnetic emissions (EME).
It is an object of this invention to provide a level shifter that allows for two substantially independent ground voltage potentials. It is a further object of this invention to provide a level shifter that has reduced electromagnetic emissions (EME), compared to the conventional level shifters of FIGS. 1 and 2.
These objects, and others, are achieved by providing a symmetric differential driving signal relative to one set of voltage potentials to a pair of current sources that drive an output node to another set of voltage potentials. The differential driving signals from the driving system are equal and opposite to each other, thereby avoiding stray current flow between the driving system and the driven current mirrors. The transistors that provide the driving signal are continuously biased, using a weak bias in one logic state and stronger bias in the other state, to avoid hard-switching transients.